Interpretive Summary: The desktop mapping software Surfer can be used as a simple geographic information system to estimate the nitrogen removed in grain and establish the variable rate fertilizer nitrogen requirements for a following crop. The method is illustrated with results from a farm field in northern Montana.

Technical Abstract:
Optical protein sensors and mass-flow yield monitors provide the opportunity to continuously measure grain quality and quantity during harvesting. This chapter illustrates how yield monitor and grain protein measurements may provide useful postharvest information for evaluating water or nitrogen (N) limitations in wheat. The surface-mapping software Surfer is used to create yield and protein maps that share a common grid, and then calculate maps of critically low protein, N removed in grain, and N management zones. Analysis of a critical spring wheat protein level provided site-specific information needed to assess where N had been adequate or deficient within an irrigated northern Montana production field. Where N was adequate (=132 g of protein kg-1 of grain), variable-rate N management could be based on replacing N at the rate at which it was removed in grain. Where it was deficient (<132 g kg-1), N management was based on university recommendations that involve yield potentials and soil nitrate-N test values.